Method for producing a pre-stressed concrete structure

ABSTRACT

A core member having a tubular body of foam material is mounted on a concrete reinforcing tendon adjacent the bulkhead of a mold. In on embodiment, the body is formed of flexible polyurethane foam and has a longitudinal extending slit which provides for flexing the body to mount it on the tendon. After the concrete has cured and the bulkhead has been removed, a torch is directed at the foam body to cut the tendon at a point recessed below the surface of the concrete with minimal hazards of flying cement particles.

Jan. 4, 1972 Q mLGEMAN, R 3,632,124

METHOD FOR PRODUCING A PRE-STRESEED CONCRETE STRUCTURE Filed April 28, 1969 l/VVE'NTOR CHARLES E. HILGEMAN JR.

A TTORNE'YS United States Patent O 3,632,724 METHOD FOR PRODUCING A PRE-STRESSED CONCRETE STRUCT Charles E. Hilgeman, In, Center-ville, Ohio, assignor to The Dayton Sure-Grip and Company, Miamisburg,

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Filed Apr. 28, 1969, Ser. No. 819,657 Int. Cl. B28!) 7/34, 23/04 US. Cl. 264-228 2 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Prestressed concrete structures are commonly formed within a generally rectangular mold which includes a horizontal stressing or casting bed bordered by two easily stripped upstanding steel side forms and two opposing steel bulkheads extending between the side forms. The bulkheads are usually anchored either to the ends of the bed or independently to the ground and have a plurality of horizontally aligned holes for receiving steel cables or tendons which extend through the mold in a predetermined pattern.

Individual tendons or cables having two or more twisted tendons may be used. The number and size of the tendons depend upon the size of the concrete structure, the prestressing demands and the modules of elasticity of the steel being used. For simplicity, the word tendon will hereafter be used as inclusive of a reinforcing rod or cable.

After the tendons have been threaded between the aligned holes in the bulkheads are pre-tensioned to suspend them above the stressing bed. The most common method of pre-tensioning the tendons is to mount hydraulic jacks on one of the bulkheads. The jacks commonly have wedge or chuck type grips which grip the ends of the tendons and pull them outwardly relative to the adjacent bulkhead thereby elongating or stretching them a short distance to effect pre-tensioning. It is not uncommon for a hydraulic jack to apply up to 90 tons of pressure on a tendon having a 1 /2 inch nominal diameter.

After the tendons have been pre-tensioned to the desired force and anchored to a metal plate affixed to the outside surface of the bulkhead, concrete is poured into the mold. When the concrete has set sufficiently to adhere to all of the tendons, the tendons are released from their anchorages. The pre-tensioning forces are thereby transferred to the structure through the steel-to-concrete bond which restrains the tendons from shortening. The jacks and bulkheads are then removed and the protruding ends of the tendons are cut off with an acetylene torch.

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The cutting of the protruding ends of the tendons from the concrete structure has presented several problems. First, the cutting of the tendons from the structure with an acetylene torch is quite hazardous. That is, cured concrete consists of a matrix of extremely strong and taut bonds between the cement particles and the sand and gravel particles. When the intense heat of the torch contacts the surface of the concrete it breaks the particle-toparticle bonds thereby releasing the bond energy and in effect causing an explosion which propels minute particles of the cement in random directions from the face of the cement. These flying particles are extremely hazardous to the person operating the acetylene torch.

Second, the tendons are preferably severed at a point recessed below the concrete surface in order that they may be buried within grouting troweled flush with the face of the concrete. Otherwise, they will protrude from the surface of the concrete and their ends will have to be coated to protect them from rusting and corroding. This results in extra fabrication steps and an unsightly struc-' ture.

In an attempt to avoid these problems, clay has been hand-packed about each pre-tensioned tendon each bulkhead to form a core member for each tendon. The clay packings are surrounded by concrete poured into the mold, after the concrete has cured and the bulkheads have been removed, the tendons will protrude from the clay packings which define cavities in the cured structure. The clay permits the torch operator to out each tendon at a point recessed from the surface of the structure and thereby minimize, to a certain extent, the contact of the flame of the torch with the concrete surface.

However, clay packings have several disadvantages. For example, the clay must be mixed with water in order to make it easy to manipulate and pack, and one or more handfuls of clay must be taken from the mixture and hand-packed or arranged about each tendon adjacent each bulkhead. This hand-packing operation requires considerable time and inherently results in an uneven and randomly shaped packing. Furthermore, the clay presents resistance to the flame which must cut through and penetrate the clay before reaching the tendon, and the heat of the flame often causes part of the clay to fly out of the recess toward the operator. Moreover, the portion of clay which remains Within the cavity must be cleaned out requiring additional time.

SUMMARY OF THE INVENTION The present invention is directed to an improved core member which facilitates cutting of a tendon extending from a prestressed concrete structure and to the method of using the member. The invention substantially reduces the time required for preparing a form for a prestressed concrete structure and also reduces the time and hazards of cutting the tendons with a torch. Generally, the core member comprises a tubular body of material capable of supporting combustion and having a bore therethrough for receiving a tendon. The body is capable of being opened to facilitate mounting of the body on the tendon.

Preferably the body is formed of a flexible polyurethane foam, and is either cylindrical or frusto-conical in shape. An a xially extending radial slit is formed within the body to provide for conveniently flexing the body for mounting on the tendon. The core member is used in the following manner. After the tendons have been pre-tensioned, a core member is mounted on each tendon adjacent each bulkhead. This is accomplished simply by grasping and flexing the member, aligning the open slit with the tendon, and then exerting a force to cause the tendon to pass through the slit.

The core member will displace a volume of concrete and remain inside the structure after the concrete has cured and the bulkheads have been removed. When a flame from an acetylene torch is directed angularly against the core member recessed in the concrete structure, the core member disintegrates and the tendon is severed at a point recessed below the concrete surface. The core member presents little or no resistance to the flame. Contact between the flame and concrete will occur only within the cavity which serves as a shield to deflect flying cement particles. Thus the core member substantially reduces the hazards involved in cutting such tendons.

The resultant prestressed concrete structure will thereafter have a plurality of uniform cylindrical or frustoconical cavities surrounding the severed ends of the tendons. These cavities may be easily filled with grouting which covers the severed ends of the tendon and fills the cavities flush with the concrete surface.

DESCRIPTION OF THE DRAWING FIG. 1 is a section view of a concrete mold and showing a pair of core members constructed in accordance with the invention and mounted on a steel tendon adjacent the inner surface of the bulkheads;

FIG. 2 is a perspective view of one of the core members shown in FIG. 1;

FIG. 3 is a fragmentary perspective view of a modified core member constructed in accordance with the invention;

FIG. 4 is an end view of the core member shown in FIG. 2 and illustrating the flexed condition as it is being mounted on a tendon;

FIG. 5 is a fragmentary perspective view of a core member within a cured prestressed concrete structure after the mold has been removed and before the tendon has been severed with an acetylene torch; and

FIG. 6 is a fragmentary sectional view taken generally on the line 66 of FIG. 5 and showing a tendon after it has been cut by a torch.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A prestressed concrete mold 10 generally includes a horizontal stressing or casting bed 12 which extends between a pair of vertical bulkheads 14. A pair of steel side forms 18 extend between the bulkheads 14, although only the rear side form 18 is shown in FIG. 1. In practice, the bulkheads 14 would have a plurality of aligned holes 19 for receiving a plurality of cables or tendons 20. For simplicity, however, a single tendon 20 is shown extending in pre-tensioned condition the length of stressing bed 12 and passing through one set of aligned holes 19 in the bulkheads 14. The tendon 20 is usually pre-tensioned by hydraulic jacks (not shown) which engage the bulkheads 14 adjacent the holes 19 and grip the end portions of the tendon 20 to pull them in the direction of the arrows 22.

In accordance with the invention, a pair of core members are mounted on the tendon 20 adjacent the inner surfaces of the bulkheads 14. Each core member 25 consists of a body 26 of flexible polyurethane foam material which is capable of supporting combustion. For convenience of forming and handling, the shape of the body 26 is preferably either cylindrical, as shown in FIG. 2, or frusto-conical as shown in FIGv 3. The body 26 is formed with an axially extending bore 28 which has a diameter slightly greater than the nominal diameter of the tendon 4 20. The body 26 is also provided with an axially extending radial slit 39 which is continuous from the bore 28 to the outer surface of the body 26 so that the body may be flexed to open the bore 23 for receiving the tendon 20.

Each of the core members 25 is mounted on a tendon 20 by grasping and flexing the body 26 (FIG. 4) to open the bore 28 for receiving the tendon 20. One of the core members 25 is positioned on each tendon 20 adjacent the inner surface of each bulkhead 14. After the concrete is poured into the mold 1t) and cures, the side forms 18 and bulkheads 14 are removed to form a structure 35 (FIG. 5) having opposite end surfaces 36.

The flame of an acetylene torch 40 (FIG. 5) is used to sever the projecting ends of each tendon 29 from the structure 35. This is accomplished by bringing the flame angularly against the core member 25 causing the core member to disintegrate before severing the tendon 20 at a point recessed below the concrete surface 36. The dimensions of the core member should be sufliciently large to permit the flame to be brought against the tendon at a point recessed below the concrete surface 36 without appreciable contact between the flame and the concrete surface 36.

When the tendon 29 is cut in this manner, the flame will quickly disintegrate the core member without any significant odor to form a corresponding cavity 45 (FIG. 6) surrounding the severed end portion of the tendon. Furthermore, the tendon 20 is cut at a point recessed below the concrete surface 36 and within the cavity 45 which acts as a shield for any flying cement particles. After each tendon 20 is severed, grouting may be conveniently applied within each cavity 45 to cover or bury the severed end of the tendon and fill in the cavity flush with the concrete surface 36. This will prevent rusting of the end portion of the tendon and give the surface 36 a pleasing appearance.

The core member of the invention thus overcomes the main problems which have previously been encountered in the production of prestressed concrete structures. For example, the invention eliminates the expensive and time consuming procedure of hand-packing clay around each tendon. Furthermore, the core member is inexpensive in construction, may be quickly and conveniently mounted on a tendon, and minimizes the hazards incident to cutting tendons from cured concrete structures with torches.

While the core members and method herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise core members and method, and that changes may be made therein without departing from the scope of the invention.

What is claimed is:

1. An improved process for producing a pre-stressed concrete structure, comprising the steps of positioning a reinforcing tendon within a mold with at least one end portion of the tendon extending through a bulkhead of the mold, pulling said end portion of the tendon to pretension the tendon, forming a tubular body of plastics foam material having a longitudinal slit therein, flexing the body of foam material and mounting the body on the tendon adjacent the inner surface of the bulkhead, pouring concrete into the mold around the tendon and tubular body, allowing the concrete to cure, releasing the pre-tension on the tendon after the concrete is cured, removing the bulkhead, directing the flame of a cutting torch against the body of foam material within the cured concrete to cause disintegration of the body and to form a cavity within the concrete surrounding the tendon, and cutting the tendon with the torch flame at a point within the cavity and recessed below the concrete surface formed by the bulkhead providing for covering the severed end of the tendon by filling the cavity with grout.

2. A process as defined in claim 1 wherein the tubular body is formed of a polyurethane foam material.

References Cited UNITED STATES PATENTS 5 9/1931 Thompson 249-40 X 7/1962 Olivier 264-228 X 10/1963 Brandestini 25-118 T X 2/1966 Vitz 264228 X 4/1966 Sabouni s2 223 R X 10 5/1968 Shaw 25-118 T FOREIGN PATENTS 269,576 10/1950 Switzerland 18Dest. Mold Dig.

1,112,253 8/1961 Germany 18Dest. Mold Core Dig.

ROBERT F. WHITE, Primary Examiner J. H. SILBAUGH, Assistant Examiner US. Cl. X.R.

52223 R; 249-40, 62; 264-317, Dig. 44 

